.NET-Community-Toolkit/CommunityToolkit.Mvvm/Messaging/Messages/RequestMessage{T}.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System;
using System.Diagnostics.CodeAnalysis;

#pragma warning disable CS8618

namespace CommunityToolkit.Mvvm.Messaging.Messages;

/// <summary>
/// A <see langword="class"/> for request messages, which can either be used directly or through derived classes.
/// </summary>
/// <typeparam name="T">The type of request to make.</typeparam>
public class RequestMessage<T>
{
    private T response;

    /// <summary>
    /// Gets the message response.
    /// </summary>
    /// <exception cref="InvalidOperationException">Thrown when <see cref="HasReceivedResponse"/> is <see langword="false"/>.</exception>
    public T Response
    {
        get
        {
            if (!HasReceivedResponse)
            {
                ThrowInvalidOperationExceptionForNoResponseReceived();
            }

            return this.response;
        }
    }

    /// <summary>
    /// Gets a value indicating whether a response has already been assigned to this instance.
    /// </summary>
    public bool HasReceivedResponse { get; private set; }

    /// <summary>
    /// Replies to the current request message.
    /// </summary>
    /// <param name="response">The response to use to reply to the request message.</param>
    /// <exception cref="InvalidOperationException">Thrown if <see cref="Response"/> has already been set.</exception>
    public void Reply(T response)
    {
        if (HasReceivedResponse)
        {
            ThrowInvalidOperationExceptionForDuplicateReply();
        }

        HasReceivedResponse = true;

        this.response = response;
    }

    /// <summary>
    /// Implicitly gets the response from a given <see cref="RequestMessage{T}"/> instance.
    /// </summary>
    /// <param name="message">The input <see cref="RequestMessage{T}"/> instance.</param>
    /// <exception cref="System.ArgumentNullException">Thrown if <paramref name="message"/> is <see langword="null"/>.</exception>
    /// <exception cref="InvalidOperationException">Thrown when <see cref="HasReceivedResponse"/> is <see langword="false"/>.</exception>
    public static implicit operator T(RequestMessage<T> message)
    {
        ArgumentNullException.ThrowIfNull(message);

        return message.Response;
    }

    /// <summary>
    /// Throws an <see cref="InvalidOperationException"/> when a response is not available.
    /// </summary>
    [DoesNotReturn]
    private static void ThrowInvalidOperationExceptionForNoResponseReceived()
    {
        throw new InvalidOperationException("No response was received for the given request message.");
    }

    /// <summary>
    /// Throws an <see cref="InvalidOperationException"/> when <see cref="Reply"/> is called twice.
    /// </summary>
    [DoesNotReturn]
    private static void ThrowInvalidOperationExceptionForDuplicateReply()
    {
        throw new InvalidOperationException("A response has already been issued for the current message.");
    }
}